Dual Nature of Radiation and Matter

A convex lens of focal length 40 cm forms an image of an extended source of light on a photoelectric cell. A current I is produced. The lens is replaced by another convex lens having the same diameter but focal length 20 cm. The photoelectric current now is :

1. I2
2. 4 I
3. 2 I
4. I

Answer

(4) I

Solution

Photocurrent light power (photons per second) falling on the cell.

same diameter same aperture area the lens collects the same light power from the source

Smaller f only makes the image smaller and brighter; the whole image still falls on the cell.

total light power on the cell is unchanged current = I

Answer: (4)